In recent years, application of devices having micro structures formed by micromachining technology is being promoted in various technical fields. Examples of such devices include micro movable devices including minute movable portions, such as micro mirror devices, angular velocity sensors, acceleration sensors, etc. Micro mirror devices are utilized as a device having a light reflection function in the fields of optical disc technology and optical communication technology, for example. Angular velocity sensors and acceleration sensors are utilized for e.g. camera shake control technology for video cameras or mobile phones equipped with cameras, car navigation systems, airbag release timing control systems and attitude control systems in automobiles and robots. Such micro movable devices are disclosed in e.g. Patent Documents 1-3 identified below:
Patent Document 1: Japanese Laid-open Patent Publication No. 2003-19700
Patent Document 2: Japanese Laid-open Patent Publication No. 2004-341364
Patent Document 3: Japanese Laid-open Patent Publication No. 2006-72252
FIGS. 23-25 illustrate a micro movable device X3 as an example of conventional micro movable device. FIG. 23 is a plan view of the micro movable device X3. FIGS. 24 and 25 are sectional views taken along lines XXIV-XXIV and lines XXV-XXV in FIG. 23, respectively.
The micro movable device X3 includes a pivotable portion 40, a frame 51, a pair of torsion bars 52 and a comb-tooth electrode 53 and is designed as a micro mirror device. To clarify the figure, the pivotable portion 40 and the frame 51 are indicated by hatching in FIG. 23.
The pivotable portion 40 includes a land portion 41, a comb-tooth electrode 42 and a bar portion 43 and is provided to be able to pivot. The obverse surface of the land portion 41 is provided with a mirror surface 41a having a light reflection function. The comb-tooth electrode 42 forms the movable electrode of the driving mechanism of this device and is made of a silicon material which is made electrically conductive. The bar portion 43 is made of a silicon material which is made electrically conductive and connects the land portion 41 and the comb-tooth electrode 42 to each other.
The frame 51 has a shape to surround the pivotable portion 40 and is made of a silicon material which is made electrically conductive.
The paired torsion bars 52 define the axis A3 of the pivot movement of the pivotable portion 40 and the land portion 41. Each of the torsion bars 52 is connected to the bar portion 43 of the pivotable portion 40 and the frame 51 to join these parts to each other. The torsion bars 52 are made of a silicon material which is made electrically conductive and function to electrically connect the frame 51 and the bar portion 43 to each other.
The comb-tooth electrode 53 is a portion to act in cooperation with the comb-tooth electrode 42 to generate electrostatic attraction. As illustrated in FIG. 25, the comb-tooth electrode 53 is fixed to the frame 51 via an insulating film 54. That is, the comb-tooth electrode 53 forms the stationary electrode of the driving mechanism of this device. The comb-tooth electrode 53 is made of a silicon material which is made electrically conductive. The insulating film 54 is made of silicon oxide and has a thickness of 0.5 μm. As illustrated in FIGS. 24 and 25, the comb-tooth electrodes 42 and 53 are positioned at different heights when the pivotable portion 40 is not in operation, for example. The spacing distance between the comb-tooth electrodes 42 and 53 in the non-operating state is about 3 μm. The comb-tooth electrodes 42 and 53 are arranged to be offset from each other so as not come into contact with each other during the pivot movement of the pivotable portion 40.
In the micro mirror device X3, the pivotable portion 40 or the land portion 41 is rotationally displaced about the axis A3 by applying a predetermined potential to each of the comb-tooth electrodes 42 and 53 as required. The application of the potential to the comb-tooth electrode 42 can be achieved via the frame 51, the paired torsion bars 52 and the bar portion 43, and the comb-tooth electrode 42 is connected to ground. When a predetermined potential is applied to each of the comb-tooth electrodes 42 and 53 to generate an electric field and hence a desired electrostatic attraction between the comb-tooth electrodes 42 and 53, the comb-tooth electrode 42 is attracted to the comb-tooth electrode 53. As a result, the pivotable portion 40 or the land portion 41 pivots about the axis A3. The pivotable portion 40 or the land portion 41 can be rotationally displaced to an angle at which the electrostatic attraction between the two electrodes and the total of the torsional resistance forces of the torsion bars 52 balance with each other. To control the amount of rotational displacement in this pivot movement, the potential application to the comb-tooth electrodes 42 and 53 is controlled. When the electrostatic attraction to act between the comb-tooth electrodes 42 and 53 is eliminated, the torsion bars 52 return to the natural state, and the pivotable portion 40 and the land portion 41 have a posture as illustrated in FIG. 25. By driving the pivotable portion 40 or the land portion 41 into the above-described pivot movement, the direction of the light reflection at the mirror surface 41a on the land portion 41 is changed appropriately.
To drive the micro movable device X3, an electric field needs to be generated by generating a potential difference between the comb-tooth electrodes 42 and 53, as described above. However, when a potential difference is generated between the comb-tooth electrodes 42 and 53, a potential difference is generated also between the frame 51, which is electrically connected to the comb-tooth electrode 42, and the comb-tooth electrode 53. Further, to properly drive the micro movable device X3, a relatively strong electric field needs to be generated between the comb-tooth electrodes 42 and 53 by generating a relatively great potential difference. However, in driving the device, an electric field stronger than this tends to be generated at the insulating film 54 intervening between the frame 51 and the comb-tooth electrode 53. This is because the dielectric constant of the insulating film 54 is higher than that of the gap between the comb-tooth electrodes 42 and 53. A stronger electric field is generated at the insulating film 54 as the thickness of the insulating film 54 becomes smaller as compared with the gap between the comb-tooth electrodes 42 and 53.
The strong electric field generated at the insulating film 54 causes the deterioration of insulation properties of the insulating film 54. Thus, in the micro movable device X3, the insulation properties of the insulating film 54, which intervenes between the frame 51 and the comb-tooth electrode 53 to bond these parts together, easily deteriorates. When the insulation properties of the insulating film 54 deteriorate, the driving characteristics of the micro movable device X3 deteriorate. When the insulation properties of the insulating film 54 deteriorates and dielectric breakdown occurs at the insulating film 54, the micro movable device X3 cannot be driven.
The present invention is proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a micro movable device and a micro movable device array which are suitable for suppressing the deterioration of the driving characteristics.